The present invention relates to a process for the preparation of anionic polyurethane latexes without the necessity of incorporating emulsifying surface active agents therein, as well as to the latexes obtained and their applications.
The development of polyurethane elastomers satisfactory for use as film-forming agents has been extremely difficult, since it was necessary to apply the elastomers in solution form in a solvent which was both costly and toxic, i.e., dimethylformamide. This has prompted numerous attempts in the art to prepare polyurethane elastomers in latex form.
In general, these latexes differ from vinyl and acrylic type latexes in that the emulsive part thereof which determines their stability must be an integral part of the molecule and, hence, cannot comprise an emulsive substance foreign to the polymer.
The usual method of obtaining such latexes consists in preparing, in the absence of water, a polyurethane prepolymer normally insoluble in water, with or without isocyanate end groups of relatively low molecular weight, so that it remains sufficiently fluid to be emulsified. In order for such a product to possess film-forming properties, it is further necessary to increase its molecular weight.
In the case of cationic latexes, the incorporation of the ionic group in the macromolecular chain does not present a difficult problem. A certain amount of an aminodialcohol soluble in the polyether or polyester which is the basic constituent of polyurethane is incorporated during the fabrication of the prepolymer; this aminoalcohol can therefore readily react with the diisocyanate.
The problem becomes more complex, however, when it is desired to prepare an anionic latex. In order to incorporate the ionic group during the fabrication of the prepolymer, it would be necessary to provide a bifunctional compound with a sodium sulfonate function soluble in the basic polyester or polyether component which is virtually impossible in practice.
Various processes for preparing anionic polyurethane latexes have been proposed. Thus, French Pat. No. 1,499,121 granted Dec. 8, 1965, proposes the preparation of a prepolymer emulsion without a foreign ionic surface active agent, the latter being introduced during mixing with water and becoming attached to the prepolymer during the chain propagation reaction.
French Pat. No. 1,580,014, Sept. 18, 1968, uses, as anionic constituents, lysine in the form of its alkaline salts, which also function as chain propagation agents by reaction with a prepolymer, and thus leading to high molecular weight polyurethanes. The foregoing lysine salts are used in such proportions that after the reaction with the polymer, the final polyurethane contains in its dry extract between 0.05 and 1% by weight of COO.sup.- groups.
Finally, French Patent Application No. 2,014,990, Apr. 24, 1970, uses presulfonation with sulfuric acid or a sulfonic acid, either of the diisocyanate, or of the prepolymer, the sulfonated prepolymer being then reacted with a base and mixed with an aqueous medium in which it undergoes chain propagation to form a polyurethane in latex form. In addition to the fact that acceptable reproducibility of the sulfonation reaction is difficult to achieve, this process does not seem to lead to homogeneous latexes with sufficiently small particles to provide for a suitable service life and film-forming properties.
Thus, each of these prior art processes, although of technical interest, nevertheless involve attendant difficulties in practice with respect to maintaining the level in the prepolymer or, in those processes wherein the emulsifying group is incorporated in an aqueous medium, there is a substantial risk that the reaction increasing the molecular weight will prevail over the desired reaction incorporating the emulsifying group in the prepolymer. Therefore, there exists a need for a simple process for the incorporation of the ionic group in the prepolymer chain to afford an emulsifiable polyurethane which will form the desired latex in water.